1. Field of the Invention
The present invention relates to a nano tip and fabrication methods of the nano tip, which is generally used in mechanical, physical, and electrical devices for detecting surface signals or chemical signals, or that may be used as a source of an energy beam. In particular, it relates to a nano tip and fabrication methods of the same that may be used as a probe needle of a scanning probe microscope (SPM) to be able to detect mechanical and chemical action, a needle-type nano probe primarily made of carbon material for application to chemical or biological sensors, or a source of an energy beam such as a cold cathode source scanning X-ray or an electron beam.
2. Description of the Related Art
A scanning probe microscope is a microscope that detects physical and chemical reaction of atoms on the surface of a sample by means of a probe tip attached to its probe. The probe tip serves as a sensor detecting the physical and chemical reaction, and is mounted at the very end of the probe. The structure of the probe depends on what physical quantity is to be detected, and generally a tip with a finer structure may detect a smaller unit of physical quantity. Also, a tip with a particular shape may carry out two-dimensional measurement rather than one-dimensional measurement. Therefore, recently, a carbon nanotube with a diameter of close to 1 nm has been used as the probe tips of the microscopes.
Of the scanning probe microscopes, there are a scanning tunneling microscope (STM) that measures tunnel current, an atomic force microscope (AFM) that detects surface indentation by using Van der Waals atomic force, a lateral force microscope (LFM) that detects a surface difference by friction force, a magnetic force microscope (MFM) that detects characteristics of a magnetic field by using a magnetized needle, an electric field force microscope (EFM) that measures an electrical field by applying a voltage between the sample and the probe, a chemical force microscope (CFM) that measures surface distribution of a chemical functional group, a scanning capacitance microscope (SCM) that measures capacitance between a sample and a probe, a scanning thermal microscope (SThM) that displays distinct images of thermal distribution on the surface, and an electrochemistry scanning probe microscope (EC-SPM) that measures electrochemical properties of a specimen, to name a few. Generally, these microscopes detect surface signals with a high resolution of up to atomic scale.
The AFM is widely used in the various fields of nano-technology from fundamental research to processing equipment for production. The probe tip is regarded as a core technology for the AFM, and the shape and size of the probe tip changes the resolution and reproducibility of images of the AFM. The AFM is widely used in the field of measurement and observation up to nanoscale, and many studies are being carried out on soft lithography using the AFM.
The AFM tip is generally a sharp pyramid-shaped tip formed at the free end of the AFM cantilever. However, the carbon nanotube may be bonded to the apex of the pyramid. This is because a tip with a very high aspect ratio and a small diameter is favorable for measurement in the atomic scale.
The carbon nanotube tip is well-known to have an ideal feature for improving the AFM in measurement, controllability, and manufacturing due to its sharpness, high aspect ratio, high mechanical stiffness, high elasticity, and controllability of chemical components.
The tip made of carbon nanotubes has a long life, is suitable for measuring a narrow and deep structure, and has the advantage of high resolution that is capable of measuring a size smaller than 1 nm.
Recently, it has been reported that multi-walled carbon nanotubes (MWNT) or single walled carbon nanotubes (SWNT) are directly grown by a chemical vapor deposition (CVD) method suggested by Hafner et al. (U.S. Pat. No. 6,346,189). For the individual growth of the AFM probe tips, this method coats catalyst particles and makes them grow in a high temperature hydrocarbon gas. By this method, a bundle of the MWNT or the SWNT is bonded to the end of the AFM.
A method developed by Dai is very effective. In the method, liquid phase precursors are coated on the end of an AFM tip and are grown by the CVD method. After that, the AFM tip attached with the carbon nanotubes is finished through the discharging process for adjusting the size (U.S. Pat. No. 6,401,526). The liquid phase precursor is made by mixing a salt-containing metal, a long-chain molecular compound, and a solvent. Also, in order to attach the phase precursor effectively, a method is provided in which a plurality of pyramid tips are coated at the same time by micro-contact printing.
In addition, it has been recently reported that the carbon nanotubes are grown by spreading a phase precursor on a spin coating at a wafer mounted with many silicon pyramid tips for the AFM, followed by removing the phase precursor by etching it except the phase precursor on the pyramids so as to leave the phase precursor on the tip of the pyramid only, and then using the CVD method in a gas containing carbon (see Wafer scale production of carbon nanotube scanning probe tips for atomic force microscopy, Applied Physics letters, Vol. 80, No. 12, Erhan Yenilmez et al., 2002, March, pp 2225-2227).
In addition, the carbon nanotubes have been bonded to the AFM tip directly with an adhesive, and Piezomax Co. has commercialized the carbon nanotube probe for the AFM by bonding a bundle of MWNTs and grinding the end thereof to be sharp.
Many studies have been carried out to detect mechanical, chemical, and biological features of the AFM probe attached with such carbon nanotubes having excellent aspect ratio and properties. However, it is very difficult to fabricate a nanoscale probe with these carbon nanotubes attached. In particular, previously reported methods take a long time for production in even a small quantity or have the disadvantage that uniform performance is not obtained in mass production. Also, it is nearly impossible to obtain the required length and diameter and to control the shape of the end of the tip. Therefore, when measuring samples, each tip may show the different result so that the tips are not suitable for industrial use.
On the whole, the previous nano tips made by multi-layering carbon have been made inside a scanning electron microscope by gradual deposition and growth at one end of the tip while flowing carbon gas. However, not only does this method requires a lot of time, but it has difficulties in making a very thin tip with a uniform diameter.
State-of-the-art semiconductors have a line width smaller than 0.1 μm, so the AFM is used to measure the width accurately in the process. However, it is difficult for the conventional probe or tip of the AFM to carry out the accurate measurement, and there is a growing need for fabricating nano tips to solve the problem.
On the other hand, the nano tips having carbon nanotubes attached on a conductor have been tested as a beam source of a scanning electron microscope or a beam source for X-rays. However, making a probe with extreme delicacy and reliability is so difficult that its commercialization has not yet been achieved.